I. Field of the Invention
The present invention relates to a process for improving adhesion characteristics of a polymeric substrate surface.
II. Brief Description of Relevant Art
Polymeric materials provide excellent and versatile mechanical qualities and find use in a wide variety of applications. In certain instances, it is necessary to coat the surface of polymeric materials to meet aesthetic requirements or to provide a protective surface to help the polymeric substrate withstand degradation. Providing a high quality durable painted surface on certain polymeric substrates has been problematic due to generally poor surface adhesion qualities exhibited by various polymeric substrate materials. Poor surface adhesion is also problematic in situations in which other laminates, films or metallic layers are to be imparted onto the polymer. Situations can also include bonding of one polymer substrate to another polymeric material or to non-polymeric substrates.
To this end, much activity has been devoted to providing polymers with improved surface adhesion characteristics and to improving the surface adhesion characteristics of known polymeric materials. Methods include roughening of polymeric surfaces by exposure to strong mineral acids with or without the presence of strong oxidizers such as chromates, permanganates and the like.
Other adhesion promotion methods involve the use of strong mineral acids in combination with concentrated mineral oxidants and strong mineral acid salts in aqueous treatment solutions. This art is taught in U.S. Pat. No. 3,869,303 to Orlov et al. Problems associated with the use of such strong mineral acids and toxic oxidizers include handling and disposal risks as well as the potential that the polymeric surface and substrates may be excessively degraded or compromised. Additionally, it is possible that the resulting polymeric substrate surface may be susceptible to unwanted oxidation or the like, necessitating immediate coating with the desired coating material.
Treatment of polyolefin materials for adhesive bonding using a non-chromate solution containing sulfuric acid in the presence of either lead dioxide, potassium iodate or ammonium persulfate is taught in U.S. Pat. No. 4,835,016 to Rosty et al. The Rosty reference also discusses the use of a solution containing bleach and detergent. In order to be effective, the Rosty reference teaches that the polymeric samples must be submersed in the prescribed solution for several days. Such protracted exposure is simply not practical in many treatment operations.
Treatment methods are also known which teach the use of peroxydisulfuric acid solutions with the optional use of accelerants capable of activating peroxydisulfuric acid oxidation reactions. In U.S. Pat. No. 3,695,915 to Morris, metal salts such as copper, ferrous or silver salts of sulfuric acid or nitric acid are employed to increase the rate of evolution of oxygen. Care must be taken in employing the process disclosed in Morris lest excess concentrations of deleterious sulfuric acid are produced.
In U.S. Pat. No. 5,05,256 to Haag, a method for improving adhesion of paints to polydicyclopentadiene is proposed. A solution containing sodium hypochlorite and, preferably, a detergent is applied to the polymer by wiping. The solution is allowed to remain on the polymer surface for a 40 minute interval and is then washed off. Polydicyclopentadiene presents a unique and exotic polymeric structure. Without being bound to any theory, it is believed that the large number of unsaturated carbon-carbon linkages in the cyclic functionalities provides a material uniquely susceptible to interaction with hypochlorite compounds. Unfortunately, the method described in Haag has not been readily adaptable to other more commonplace polymers.
Heretofore, no method has been proposed which promotes surface adhesion characteristics of a broad range of polymeric substrates in an efficient, economical manner which reduces reliance on chemical compounds which present handling and disposal difficulties; and, it is an object of the present invention to provide such a method. Since many of the compounds previously suggested for polymeric adhesion promotion are costly, difficult to obtain, or present handling or disposal problems, it is a further object of the present invention to provide a method for improving surface adhesion characteristics of common polymeric substrates which employs relatively low-cost chemical commodities in an easily handled fluid medium such as an aqueous solution which does not require undue special handling considerations. It is also desirable that the method yield relatively consistent treatment results and be easy to implement and monitor in a plant or manufacturing setting. It is desirable that improvements in polymeric surface adhesion be accomplished in a rapid and uniform manner over the entire targeted polymeric surface area. Finally, it is desirable that the adhesion improvement method be one which yields sufficiently permanent improvement in adhesion characteristics without unduly compromising other performance characteristics of the polymer.